Cartridge/canister fuel pumps constitute an integrated fuel pump system that permits installation and removal of the cartridge from the canister as a unit. A typical use of cartridge/canister fuel pumps is for aircraft fuel systems.
Aircraft fuel pumps generally are fuel tank mounted. In one configuration, the fuel pump may be installed entirely inside the fuel tank. Alternatively, the fuel pump may penetrate the fuel tank wall with part of the pump, the canister, being mounted inside the fuel tank and part of the pump, the cartridge or a portion thereof, passing through a penetration in the fuel tank between inside and outside of the fuel tank with an inside connection to the canister. The canister generally contains fluid connections and related fluid pathways, and various valves (e.g., check valves, service shutoff valves, pressure relief valves, pressure regulating valves, or sequence valves) for controlling the fuel flow. The cartridge generally contains a motor, bearings, and the pumping components, which tend to be more subject to wear as compared to the canister components. The cartridge/canister configuration is preferred in many applications since wear components of the cartridge can be removed and replaced without draining or entering the fuel tank. Fuel tank mounted fuel pumps generally utilize centrifugal pumping elements which typically are either radial or mixed flow type impellers.
A typical pump performance requirement includes a minimum pressure at a specific flow rate. In some fuel systems, pump output pressure at reduced flow rates is limited to a predetermined maximum or threshold value, thus resulting in minimum and maximum pressure requirements. Frequently, the performance characteristic of a centrifugal pump results in increasing pressure at flow rates below the required flow rate. Accordingly, to stay below the maximum threshold pressure requirement, a pressure relief valve is utilized to route pump discharge flow to a tank or to the pump inlet resulting in reduced output pressure.
A conventional approach for pressure relief has been to extend the fuel flow pathway to a spring loaded pressure relief valve that tends to be external to the cartridge. As a result, the discharge flow has to travel a substantial distance to be upstream of the impeller to cycle the excess discharge flow back into the impeller inlet. To accommodate such an extended flow configuration, significant extra plumbing is required. The extra plumbing increases the size, weight, and number of components of the fuel pump. Particularly for aircraft, which may have as many as eight or more cartridge/canister fuel pumps (depending upon the particular aircraft model), the extra size and weight negatively affects fuel efficiency and performance of the aircraft. Conventional pressure relief systems, therefore, have been deficient.